A generic error occurred in GDI+

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I am trying to upload a picture to a specific folder. I have a code below;
_______________ _______________ _______________ ______________
System.Drawing. Image oImg =
System.Drawing. Image.FromStrea m(TextBoxFileUp load.PostedFile .InputStream);//
..FromFile(file FullName);

System.Drawing. Image oThumbNail;// = new Bitmap(rect.Wid th, rect.Height,
oImg.PixelForma t);

Graphics oGraphic;
try
{
oThumbNail = new Bitmap(rect.Wid th, rect.Height, oImg.PixelForma t);
oGraphic = Graphics.FromIm age(oThumbNail) ;
}

oGraphic.Compos itingQuality = CompositingQual ity.HighQuality ;
oGraphic.Smooth ingMode = SmoothingMode.H ighQuality;
oGraphic.Interp olationMode = InterpolationMo de.HighQualityB icubic;
oGraphic.DrawIm age(oImg, rect);

oThumbNail.Save (destFolder + "\\" + fileName,
System.Drawing. Imaging.ImageFo rmat.Jpeg);

_______________ _______________ _______________ ______________

And a have all permissions for the specified folder for IIS_WPG, ASP.NET
Machine Account, NETWORKSERVICE and even Everyone with full control.

But when I try to save a with the code on the last line, I get an exception
message "A generic error occurred in GDI+".

What else should I do?

Thanks a lot..